The present embodiments relate to magnetic resonance imaging and to a transmit antenna selector of a magnetic resonance imaging system.
Magnetic resonance imaging (MRI) is a technique using magnetic resonance phenomena for imaging. The principle of magnetic resonance phenomena involves a nucleus containing a single proton (e.g., the proton of a hydrogen nucleus) that is present throughout the human body. The nucleus has a spinning movement, like a small magnet. The spin axes of these small magnets do not have a certain arrangement. If an external magnetic field is applied, the small magnets are rearranged according to the magnetic force lines of the external magnetic field. The small magnets are arranged in two directions that are parallel or antiparallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field may be a positive longitudinal axis, and the direction antiparallel to the magnetic force lines of the external magnetic field is called a negative longitudinal axis. The nucleus has a longitudinal magnetization component, and the longitudinal magnetization component has both direction and amplitude. Nuclei in the external magnetic field are excited by a radio-frequency (RF) pulse with a specific frequency to make the spin axes of the nuclei deviate from the positive longitudinal axis or the negative longitudinal axis to produce resonance (i.e., the magnetic resonance phenomenon). After the spin axes of the excited nuclei deviate from the positive longitudinal axis or the negative longitudinal axis, the nuclei have a transverse magnetization component. After transmission of the RF pulses is stopped, the excited nuclei transmit echo signals to release the absorbed energy discretely in the form of electromagnetic waves, the phase and energy level of the nuclei both recovering to the state before being excited. An image may be reconstructed after the echo signals transmitted by the nuclei are subjected to further processing such as space encoding.
An MRI system may operate with a number of various RF antennas (which may be referred to as coils hereinafter). The RF antennas are used for transmitting and receiving RF pulses so as to excite nuclei to radiate magnetic resonance signals and/or for collecting induced magnetic resonance signals. The MRI system includes various coils, such as a body coil covering the whole body area, a receiving coil only covering some parts of the body, and so on. An MRI system may have a large integrated coil (e.g., a body coil) permanently fixed in a magnetic resonance scanner. The integrated coil may be arranged in a cylindrical manner surrounding a patient acquisition cavity (e.g., using a structure referred to as a birdcage structure). In the patient acquisition cavity, a patient is supported on a bed (e.g., a patient positioning table) during measurement.
A transmit antenna selector (TAS) is provided to switch and transmit radio-frequency signals to two output paths, a local coil and a body coil. Past transmit antenna selectors have used mechanical switches. The mechanical switches are controlled by direct current signals. Because the total number of output paths is two, the TAS uses two mechanical switches, leading to higher costs. In addition, the service life of the mechanical switch, depending on the number of on/off times, is relatively limited.